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Locust Borer
Insect Profile and Management


Insect Profile

What is the Locust Borer?

The locust borer has been known about for hundreds of years. It’s been causing damage in the western US since at least the 1960s [1]. Adults are black- and yellow-striped, 3/4 inch long, and they are classified as long horned beetles because they have long antennae [1]. The damage is caused by larval tunneling and feeding.

What types of trees are susceptible?

Only Black Locust and its cultivars like Purple Robe Black Locust and Twisted Baby® Black Locust are affected [1]. Honey Locust is unaffected [1].The most susceptible black locusts are those experiencing stress from predisposing factors such as mechanical damage, drought stress, nutritional stress, poor soil health, and are smaller than 6” in diameter at breast height (DBH)[1]. Trees greater than 8” DBH are generally safe when they aren’t suffering from stress [1].

Consequences and Severity

Locust Borer larvae feed and create deep tunnels 3-4” deep up into sapwood and heartwood, which results in broken and dead limbs, weakened trees, excessive sprout production, and tree death [1].


Insect life cycle

Adults emerge in late summer or early fall, mate, and begin laying eggs [1]. They can be seen feeding on the pollen of flowers in the mornings in September [1]. Peak egg-laying is late september. Females deposit hundreds of white, oval eggs into bark crevices and around wounds on trunk or large branches [1]. Small white larvae hatch in about a week, and feed on inner bark and wood as they begin boring into trees. There, they overwinter (not feeding) [1]. They begin feeding again in early spring [1]. As they grow, they often return to the surface to enlarge their tunnels [1]. Larvae reach their full size of about 1 inch in mid-July to mid-August, then pupate (not feeding), before emerging as the new adults [1]. There is only one generation per year [1].


Diagnostic Clues

In spring, from the start of bud swell, look for gummosis (oozing sap) and wet spots on bark where larvae are feeding [1] . In summer, look for yellow frass when they reach the heartwood [1]. Frass is a mixture of sawdust and excrement that can be found on the bark and around the base of a tree. In fall, look for small, white eggs, and young larvae feeding stains (yellowed wood), which fades to brown [1]. New wood tissue appears as a knotty, gnarled swelling [1].




Cultural and Mechanical Control

Prune out heavily infested branches to reduce spread, and reduce risk of branch failure. Maintain tree health to reduce stress. Healthy, vigorous trees have little borer damage [1].


Biological Control

Biological control is the use of natural predators to suppress populations. Downy Woodpecker, Hairy Woodpecker, Wheelbug are natural predators of Locust Borer [1]. To utilize this form of management, avoid contact insecticides that can harm wheel bugs, and retain habitat and nesting opportunities for woodpeckers if possible.


Chemical Control


In situations where cultural, mechanical, and biological control may not be enough, it is recommended to treat with systemic insecticides. Systemic insecticides are injected or sprayed on a small area of bark at the base of a tree and translocated by a tree’s vascular system to protect the entire tree. These insecticides are much safer for the environment and the applicator, and have longer control periods. Longer control means that timing is somewhat less important and reapplications are needed less frequently.


Previously, contact insecticides were recommended but many have been discontinued and should be considered obsolete.The Utah State University (USU) Locust Borer Fact Sheet is one source, but it hasn’t been revised since 2002 to include new evidence of the efficacy of systemic insecticides [1]. Contact insecticides are applied by spraying, have a greater risk of pesticide drift and runoff, and require reapplications every 10-14 days which increases the risk to bees and other pollinators.

Systemic insecticides

There are three systemic insecticides effective against the Locust Borer that we use and recommend based on factors such as tree size and timing. Timing and application methods are chosen to prevent harm to pollinators and beneficial insects, and must be used by a licensed applicator.


1. Arborjet’s IMA-jet (containing the active ingredient imidacloprid) can be trunk injected as a preventative treatment before infestation occurs [3], and with some effectiveness against insects actively feeding on bark and sapwood [3][4]. It should be applied once per year [4]. It can only be used on trees larger than 5” DBH.


2. Zylam (containing the active ingredient dinotefuran) can be applied as a bark spray for preventative treatment [3][5], and against insects actively feeding on bark and sapwood [3][5]. It is effective 21 days after application and for 1-2 years [5]. We use Zylam on trees smaller than 5” DBH.


3. Arborjet’s ACE-jet (containing the active ingredient acephate) is a trunk injectable insecticide effective against actively feeding insects [1][2], and has the quickest response time [6]. It has been reported by some applicators to be effective within hours of treatment. Reapplications may be necessary. We use it to treat trees larger than 5” DBH.




1. Locust Borer Fact Sheet No. 35 by Dr. Jay B Karren, Extension Entomologist Revised January 2002 Alan H. Roe, Insect Diagnostician


2. Arborjet. Revolutionary Plant Health Solutions.


3. Locust Borer Authored by Theresa A. Dellinger, Diagnostician, and Eric Day, Lab Manager, Insect Identification Lab, Department of Entomology, Virginia Tech


4. IMA-jet Label.


5. Zylam Tech Brochure.

6. ACE-jet Label.

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